This invention relates to steam generators for nuclear power plants and more particularly to vertical steam generators having a feedwater inlet structure disposed in the upper portion thereof.
Vertical steam generators include a tube bundle positioned within a shell structure and encircled by a wrapper. An annular passage is formed between the wrapper and the shell. Feedwater which is introduced into the upper portion of the shell travels down the annular passage and up through the tube bundle where it is heated by primary fluid passing through the tubes. In certain steam generators, the feedwater is introduced into a feedwater inlet ring positioned above the tube bundle. Initial feedwater ring designs discharged the feedwater through holes in the bottom of the ring. This created a potential steam trap in the top of the ring that could lead to bubble collapse waterhammer, which is a rapid depressurization of a steam void caused by the introduction of cold water into the void. That feedwater ring design was modified to provide for top discharge through J-nozzles. This permitted steam to rise out of the ring and also retained the desirable feature of directing feedwater toward the tube sheet in the wrapper/shell annulus thereby distributing feedwater in desired proportions at the feed ring elevation. Examples of bottom discharge and J-tube feed rings can be found in U.S. Pat. Nos. 4,037,569, issued July 26, 1977, to Bennett et al, and 3,991,720, issued Nov. 16, 1976, to Byerley, respectively.
In either feedwater ring design, feedwater line and nozzle flow stratification can occur due to thermal and hydraulic mechanisms. The thermal mechanism involves the heating of feedwater along the feed ring. Once the feedwater enters the steam generator, it receives heat through ring wall heat conduction. Then the buoyancy force becomes significant such that in a J-tube feed ring, feedwater may discharge through some of the J-tubes while hot water enters the ring through other J-tubes. The hydraulic mechanism is the main factor for feed ring flow stratification in feed rings with bottom hole discharge. In that case, feedwater enters the steam generator and discharges through some of the bottom holes while hot water gets into the ring via other bottom holes.